R.NCS.T cheat sheet

Motor conduction: supramaximal stimulus, record compound muscle action potential (CMAP); measure distal latency, amplitude (baseline to negative peak), and conduction velocity between two stimulation sites.

Conduction velocity (m/s) = distance (mm) between stimulation sites ÷ (proximal latency − distal latency in ms). Distance is measured along the nerve, not straight-line skin.

Sensory conduction: record sensory nerve action potential (SNAP); calculate velocity from onset latency, or use peak latency for amplitude comparisons. Antidromic and orthodromic techniques give comparable velocities.

Upper limb temperature effect: velocity falls roughly 2 m/s per 1 °C cooling below 34 °C; distal latency and duration prolong. Warm limbs before testing.

Late responses: F-wave latency reflects proximal motor conduction and is roughly 50% of limb length; H-reflex is a monosynaptic sensory-motor loop, best recorded from soleus with tibial nerve stimulation.

Amplitude reflects number of excitable axons and their synchrony; latency reflects conduction time; temporal dispersion widens and lowers the CMAP with proximal stimulation.

Normal median-versus-ulnar comparison rules: median distal sensory latency exceeding ulnar by >0.3–0.5 ms, or median-versus-radial differences, support median neuropathy at the wrist.

Needle EMG complements NCS: fibrillation potentials and positive sharp waves indicate active denervation; large polyphasic motor units indicate reinnervation.

Axonal loss reduces amplitude with relatively preserved velocity; demyelination slows velocity, prolongs latency, and may produce conduction block or dispersion.

Median nerve SEP: stimulate at wrist, record Erb point (N9), cervical spine (N13), and scalp over contralateral parietal cortex (N20); N9–N13 interpeak reflects proximal plexus/root conduction.

Posterior tibial SEP: stimulate at ankle, record popliteal fossa, lumbar spine, and scalp (P37); useful for lumbosacral root and spinal cord assessment.

N20 latency is the standard scalp measure for upper limb SEP; side-to-side latency asymmetry >0.5–1.0 ms is generally abnormal.

Amplitude of cortical SEP is highly variable between subjects; latency and interpeak intervals are more reliable than absolute amplitude.

SEPs assess large-fiber dorsal column–medial lemniscus pathway; they do not evaluate pain/temperature (spinothalamic) fibers.

Anesthetic and sedative agents (especially volatile agents) prolong SEP latencies and reduce amplitudes; total intravenous anesthesia is preferred for monitoring.

During scoliosis or spinal surgery, a >50% amplitude drop or >10% latency increase in cortical SEP warrants warning the surgeon.

Stimulation rate commonly 2–5 Hz; averaging 200–500 responses improves signal-to-noise ratio; impedance kept low and balanced.

TMS over motor cortex activates corticospinal tract transsynaptically; record motor evoked potential (MEP) from a contralateral limb muscle with surface electrodes.

Resting motor threshold is the minimum stimulator intensity eliciting MEPs ≥50 µV in about half of trials; it indexes cortical excitability.

Central motor conduction time = cortical latency − (spinal/peripheral latency), isolating the intracranial and spinal cord segment.

Single-pulse TMS is generally safe; contraindications include implanted electronic devices (pacemakers, cochlear implants), intracranial metal, and epilepsy history.

Repetitive TMS can induce seizures; screening for epilepsy, prior head trauma, and medications lowering seizure threshold is required.

During spine surgery, transcranial electrical MEPs monitor corticospinal integrity; a >50–80% amplitude reduction is a warning criterion, and anesthetic effects must be controlled.

MEP amplitude is highly variable trial-to-trial; facilitation (voluntary contraction) increases amplitude and lowers threshold.

Magnetic stimulation does not directly excite deep brain structures; the coil orientation and figure-of-eight geometry shape the induced field.

Myelinated fibers: saltatory conduction at nodes of Ranvier; large-diameter fibers (A-alpha) conduct fastest, small unmyelinated C fibers slowest.

Fiber classes: A-alpha (motor/proprioception), A-beta (touch), A-delta (pain/temperature, thinly myelinated), C (pain/temperature, unmyelinated).

Median nerve: formed by C6–T1 roots from medial and lateral cords; passes carpal tunnel; supplies thenar muscles and first three and a half digits.

Ulnar nerve: C8–T1; passes cubital tunnel and Guyon canal; supplies most intrinsic hand muscles and the fifth digit.

Radial nerve: C5–T1; spiral groove; posterior interosseous branch; wrist drop with high lesions.

Common peroneal nerve: from sciatic (L4–S2); winds around fibular neck; foot drop with compression at the fibular head.

Tibial nerve: L4–S2; tarsal tunnel; supplies plantar foot and calf; H-reflex loop uses tibial sensory afferents and soleus motor fibers.

Neuromuscular junction: acetylcholine release, acetylaminoreceptor binding, endplate potential; repetitive stimulation shows decrement in myasthenia gravis.

Wallerian degeneration follows axonal injury; conduction fails distal to the lesion within days, and reinnervation proceeds at roughly 1 mm/day.

Differential amplifier with common-mode rejection ratio (CMRR) ≥80–100 dB rejects 50/60 Hz mains noise; keep electrode impedances low and balanced.

Sensitivity (gain) sets vertical display scale (µV/div); sweep speed (time base) sets horizontal scale (ms/div); filters set the frequency band.

Motor NCS filter settings commonly 2 Hz–10 kHz; sensory NCS 20 Hz–2–3 kHz; SEP 1–3 kHz; too-narrow filters distort latency and amplitude.

Stimulus artifact is reduced by adjusting ground placement, twisting leads, and using constant-current stimulators with short pulse duration (0.1–0.2 ms).

Aliasing occurs when sampling rate is below twice the highest signal frequency (Nyquist); use adequate sampling and anti-aliasing filters.

Averaging improves signal-to-noise ratio by roughly the square root of the number of trials; reject trials containing artifact.

Latency is measured to onset (motor) or onset/peak (sensory); amplitude is baseline-to-peak or peak-to-peak as specified by protocol.

Electrode impedance should generally be <5 kΩ; high or unequal impedance increases noise and artifact.

Electrical stimulation is generally avoided over the carotid sinus, the heart in patients with pacemakers, and the anterior neck; use caution with implanted devices.

Needle EMG requires attention to bleeding risk: anticoagulation, thrombocytopenia, and coagulopathy increase hematoma risk; avoid deep muscles in these patients.

Universal precautions and sterile technique for needle electrodes; single-use needles reduce transmission risk; dispose of sharps properly.

Informed consent: explain purpose, risks (discomfort, bleeding, seizure risk with TMS), and alternatives; document the discussion.

TMS safety: screen for epilepsy, implants, and metal; keep emergency seizure protocol available; avoid in pregnancy without risk-benefit discussion.

Confidentiality and accurate documentation: report raw data and technical limitations; never alter or fabricate waveforms or values.

Scope of practice: perform only procedures within training and credentialing; refer ambiguous findings for physician interpretation.

Billing and coding integrity: report only services actually performed; avoid upcoding and duplicate billing.

Carpal tunnel syndrome: prolonged median distal sensory latency, reduced median SNAP amplitude, prolonged median distal motor latency, and sometimes denervation in thenar muscles.

Ulnar neuropathy at the elbow: slowed conduction velocity across the elbow segment with focal slowing; inching studies localize the lesion.

Radial neuropathy at the spiral groove: conduction block or slowing across the segment; preserved distal latency if lesion is proximal.

Peroneal neuropathy at the fibular head: focal slowing or conduction block across the fibular neck; reduced peroneal CMAP.

Guillain-Barré syndrome: early demyelinating features-prolonged distal latencies, slowed conduction, conduction block, prolonged F-waves; amplitude relatively preserved early.

Diabetic polyneuropathy: length-dependent axonal loss-reduced SNAP and CMAP amplitudes, mild slowing; lower limbs affected first.

Amyotrophic lateral sclerosis: widespread active denervation and reinnervation with normal sensory studies; motor amplitudes reduced.

Myasthenia gravis: repetitive nerve stimulation shows >10% decrement; single-fiber EMG shows increased jitter.

SEP monitoring during carotid endarterectomy or aneurysm clipping detects cortical ischemia; significant amplitude reduction prompts shunt or blood pressure adjustment.

Spinal cord monitoring combines SEP (dorsal columns) and MEP (corticospinal tracts); combined monitoring improves sensitivity to cord ischemia.

Intraoperative MEP warning criterion: amplitude reduction >50–80% from baseline; SEP criterion: amplitude reduction >50% or latency increase >10%.

Anesthetic effects: propofol and opioids preserve MEPs better than volatile agents; neuromuscular blockade abolishes MEPs and must be minimized.

SEP in coma or brain death: absent cortical N20/P37 with preserved peripheral responses suggests poor prognosis.

TMS in rehabilitation research measures corticospinal excitability and plasticity; resting motor threshold and MEP amplitude are key metrics.

Technical confounders: limb temperature, blood pressure, and patient positioning alter evoked potentials; document and control them.

Multimodality monitoring reduces false negatives; when one modality is lost, verify technical causes before attributing to neural injury.

Brachial plexus: roots C5–T1, trunks (upper, middle, lower), divisions, cords, and terminal branches; lesions localized by pattern of muscle involvement.

Lumbosacral plexus: L1–S4; sciatic nerve divides into tibial and common peroneal branches; peroneal fibers are more vulnerable to stretch and compression.

Nerve fiber diameter determines conduction velocity; demyelination reduces velocity and produces conduction block; axonal loss reduces amplitude.

Neuromuscular junction transmission safety factor; repetitive stimulation and single-fiber EMG assess presynaptic and postsynaptic disorders.

Sensory receptors: muscle spindles (Ia) mediate stretch reflexes; Golgi tendon organs (Ib) sense tension; cutaneous receptors mediate touch and pain.

Autonomic fibers (B and C) are small and unmyelinated or thinly myelinated; they are tested by quantitative sudomotor axon reflex testing and heart rate variability.

Nerve regeneration: axonal sprouts grow at ~1 mm/day; Tinel sign advances as regeneration proceeds.

Compression neuropathy pathophysiology: ischemia and focal demyelination at sites of entrapment; early decompression restores conduction.

Common-mode rejection: a differential amplifier subtracts the two inputs; equal noise at both inputs is rejected, so balanced impedances are critical.

Filter settings define the passband; high-pass filters remove low-frequency baseline wander; low-pass filters remove high-frequency noise but can blunt sharp peaks.

Stimulus artifact can be minimized by using a constant-current stimulator, short pulse duration, and proper ground placement between stimulator and recording electrodes.

Averaging reduces random noise; the number of sweeps needed depends on signal-to-noise ratio; artifact rejection prevents contamination.

Latency measurement conventions: onset latency for motor and sensory responses; peak latency for some sensory and SEP measures; document the convention.

Amplitude is measured baseline-to-negative-peak for CMAP and SNAP; peak-to-peak is used in some SEP protocols.

Nyquist theorem: sampling rate must exceed twice the highest frequency component to avoid aliasing.

Electrical safety: leakage current limits, ground integrity, and isolation of patient-connected circuits prevent microshock.

Electrical stimulation safety: avoid stimulating across the chest in patients with cardiac pacemakers or defibrillators; consult device manufacturer guidance.

Needle EMG in patients on anticoagulants: assess INR/platelets; avoid deep or highly vascular muscles; apply prolonged pressure after withdrawal.

Infection control: hand hygiene, gloves, single-use needles, and proper sharps disposal are mandatory for every needle study.

TMS safety: screen for epilepsy, intracranial implants, and cochlear implants; provide ear protection; have a seizure management plan.

Informed consent and shared decision-making: disclose purpose, risks, benefits, and alternatives; document patient questions and consent.

Documentation standards: record raw waveforms, technical parameters, and limitations; avoid interpretive claims beyond the data.

Professional boundaries and conflicts of interest: disclose financial relationships; do not let reimbursement influence test selection.

Scope of practice and supervision: trainees must be supervised; technologists should not independently interpret studies.

Submaximal stimulation underestimates amplitude and falsely prolongs latency; verify supramaximal response by increasing stimulus until response plateaus.

Co-stimulation of adjacent nerves (e.g., median and ulnar at the wrist) produces anomalous amplitudes; use selective stimulation and check for volume conduction.

Anomalous innervation: Martin-Gruber (median-to-ulnar forearm) and accessory peroneal nerve can distort amplitude comparisons.

Temperature must be controlled; cold limbs slow conduction and prolong latencies; warm to ≥32 °C (ideally 34 °C) before testing.

Electrode placement errors (off the motor point or nerve) reduce amplitude; use anatomical landmarks and check reproducibility.

Distance measurement errors directly alter calculated velocity; measure along the nerve path with a tape, not straight-line skin distance.

Temporal dispersion with proximal stimulation widens and lowers the CMAP; compare proximal and distal amplitudes to detect conduction block.

F-wave persistence and latency variability require multiple stimuli; use at least 10 responses to derive minimum latency.

SEP amplitude is highly variable; latency and interpeak intervals are more reliable for interpretation.

Cortical SEP attenuation with preserved peripheral and spinal responses suggests central conduction delay or cortical lesion.

MEP amplitude variability requires baseline averaging and facilitation; a single trial is insufficient for interpretation.

Anesthetic agents and neuromuscular blockers confound evoked potential monitoring; document agents and doses.

Warning criteria: SEP >50% amplitude drop or >10% latency increase; MEP >50–80% amplitude reduction.

Artifact sources: electrical interference, movement, and stimulus artifact; verify with repeat trials and technical checks.

Evoked potentials assess large-fiber sensory pathways and corticospinal tracts; they do not evaluate pain/temperature fibers.

Temperature, blood pressure, and positioning alter evoked potentials; control and document these variables.

Median neuropathy at the wrist (carpal tunnel): thenar wasting, sensory loss in first three and a half digits; NCS shows prolonged median distal latency.

Ulnar neuropathy at the elbow: intrinsic hand weakness, sensory loss in fifth digit; NCS shows focal slowing across the elbow.

Radial neuropathy: wrist drop; NCS shows conduction block or slowing in the spiral groove segment.

Peroneal neuropathy at the fibular head: foot drop; NCS shows focal slowing or block across the fibular neck.

Tibial neuropathy (tarsal tunnel): plantar sensory loss and intrinsic foot weakness; NCS shows prolonged tibial distal latency.

Guillain-Barré syndrome: demyelinating features-prolonged distal latencies, slowed conduction, conduction block, prolonged F-waves.

Diabetic polyneuropathy: length-dependent axonal loss-reduced SNAP and CMAP amplitudes, mild slowing.

Amyotrophic lateral sclerosis: widespread active denervation and reinnervation; sensory studies normal.

Electrode impedance <5 kΩ; balanced impedances improve common-mode rejection.

Motor NCS filters: 2 Hz–10 kHz; sensory NCS: 20 Hz–2–3 kHz; SEP: 1–3 kHz.

Sensitivity (gain) sets vertical scale; sweep speed sets horizontal scale; document both.

Stimulus pulse duration 0.1–0.2 ms; constant-current stimulator preferred.

Ground electrode placed between stimulator and recording electrodes to reduce artifact.

Averaging improves signal-to-noise ratio; reject trials with artifact.

Latency measured to onset (motor) or onset/peak (sensory); amplitude baseline-to-peak or peak-to-peak per protocol.

Sampling rate must exceed twice the highest frequency (Nyquist) to avoid aliasing.

Avoid stimulation over the carotid sinus, anterior neck, and across the chest in patients with cardiac devices.

Needle EMG in anticoagulated patients: assess bleeding risk; avoid deep muscles; apply pressure after withdrawal.

Single-use needles and proper sharps disposal reduce transmission risk.

Informed consent: disclose purpose, risks, benefits, and alternatives; document.

TMS screening: epilepsy, implants, metal; have seizure protocol available.

Documentation: record raw data, technical parameters, and limitations; never fabricate or alter waveforms.

Scope of practice: perform only procedures within training and credentialing.

Billing integrity: report only services performed; avoid upcoding.

Median motor distal latency typically <4.2 ms; ulnar <3.5 ms; peroneal <5.5 ms; tibial <5.8 ms (lab-specific).

Median sensory distal latency <3.5 ms; ulnar <3.2 ms; sural <4.5 ms (lab-specific).

Median-versus-ulnar sensory latency difference >0.3–0.5 ms supports carpal tunnel syndrome.

Conduction velocity: median forearm >49 m/s; ulnar forearm >49 m/s; peroneal >40 m/s; tibial >40 m/s (lab-specific).

F-wave minimum latency roughly 50% of limb length; prolonged F-wave suggests proximal slowing.

H-reflex: tibial stimulation, soleus recording; prolonged latency suggests S1 radiculopathy.

Amplitude: CMAP >4 mV (median), >5 mV (ulnar); SNAP >10 µV (median), >8 µV (ulnar) (lab-specific).

Temperature correction: ~2 m/s per 1 °C below 34 °C.

Median SEP N20 latency typically ~18–20 ms; side-to-side asymmetry >0.5–1.0 ms abnormal.

Posterior tibial SEP P37 latency typically ~35–40 ms; asymmetry >1.0 ms abnormal.

N9–N13 interpeak reflects proximal plexus/root conduction; prolongation suggests root or plexus lesion.

SEP amplitude highly variable; latency and interpeak intervals more reliable.

MEP central motor conduction time typically <8 ms (upper limb) and <15 ms (lower limb) (lab-specific).

Resting motor threshold ~40–50% of maximum stimulator output (device-specific).

Warning criteria: SEP >50% amplitude drop or >10% latency increase; MEP >50–80% amplitude reduction.

Anesthetic and temperature effects must be controlled and documented.

A-alpha: largest, myelinated; motor and proprioception; fastest conduction.

A-beta: myelinated; touch and pressure.

A-delta: thinly myelinated; pain and temperature; slower.

C fibers: unmyelinated; pain and temperature; slowest.

B fibers: preganglionic autonomic; lightly myelinated.

Conduction velocity proportional to fiber diameter and myelination.

Demyelination slows velocity and produces conduction block; axonal loss reduces amplitude.

Neuromuscular junction safety factor; repetitive stimulation and single-fiber EMG assess transmission.

50/60 Hz interference: check ground, electrode impedance, and lead placement; use notch filter cautiously.

Stimulus artifact: adjust ground, twist leads, use short pulse duration.

Movement artifact: secure electrodes and relax patient.

High impedance: replace electrodes, clean skin, apply conductive paste.

Aliasing: increase sampling rate or use anti-aliasing filter.

Filter distortion: overly narrow filters blunt peaks and shift latencies.

Co-stimulation: use selective stimulation and check for volume conduction.

Document technical limitations in the report.

Leakage current limits and isolation of patient-connected circuits prevent microshock.

Avoid stimulation across the chest in patients with pacemakers or defibrillators.

TMS contraindications: implanted electronic devices, intracranial metal, epilepsy history.

Repetitive TMS can induce seizures; screen and have protocol.

Needle EMG bleeding risk: anticoagulation, thrombocytopenia, coagulopathy.

Infection control: hand hygiene, gloves, single-use needles, sharps disposal.

Temperature control: warm limbs before NCS.

Document safety screening and consent.

F-wave: supramaximal stimulation, record from muscle; reflects proximal motor conduction; minimum latency from ≥10 trials.

H-reflex: submaximal stimulation, tibial nerve, soleus recording; monosynaptic loop; prolonged latency suggests S1 radiculopathy.

F-wave persistence and latency variability require multiple stimuli.

H-reflex absent in S1 radiculopathy; F-wave may be normal.

F-wave latency roughly 50% of limb length.

H-reflex useful for proximal nerve and root assessment.

Late responses complement routine NCS.

Temperature affects late response latencies.

SEP assesses dorsal column–medial lemniscus pathway (large-fiber touch/proprioception).

MEP assesses corticospinal tract.

SEP does not evaluate pain/temperature (spinothalamic) fibers.

Combined SEP and MEP monitoring improves sensitivity to cord ischemia.

Median SEP: N9, N13, N20.

Posterior tibial SEP: popliteal, lumbar, P37.

Anesthetic effects: volatile agents prolong SEP latencies and reduce amplitudes.

Document technical and physiologic confounders.

Brachial plexus: C5–T1 roots, trunks, divisions, cords, terminal branches.

Lumbosacral plexus: L1–S4; sciatic divides into tibial and common peroneal.

Peroneal fibers more vulnerable to stretch and compression.

Carpal tunnel: median nerve entrapment.

Cubital tunnel: ulnar nerve entrapment.

Fibular head: common peroneal entrapment.

Tarsal tunnel: tibial nerve entrapment.

Compression causes ischemia and focal demyelination.

Differential amplifier subtracts inputs; equal noise rejected (CMRR).

CMRR ≥80–100 dB rejects mains noise.

High-pass filters remove baseline wander; low-pass remove high-frequency noise.

Overly narrow filters distort latency and amplitude.

Averaging improves SNR by roughly square root of trials.

Artifact rejection prevents contamination.

Nyquist: sampling rate >2× highest frequency.

Document filter settings and sampling rate.

Informed consent: purpose, risks, benefits, alternatives; document.

TMS consent includes seizure risk.

Needle EMG consent includes bleeding and discomfort.

Document raw waveforms and technical parameters.

Never fabricate or alter data.

Disclose conflicts of interest.

Respect confidentiality.

Refer ambiguous findings for physician interpretation.

Radiculopathy: normal SNAP (dorsal root ganglion spared) with abnormal CMAP and EMG in myotomal distribution.

Plexopathy: abnormal SNAP and CMAP in plexus distribution; EMG shows denervation.

F-wave and H-reflex help localize proximal lesions.

S1 radiculopathy: absent H-reflex.

C5–C6 radiculopathy: abnormal median/radial SNAP may be normal; EMG key.

Plexus lesions: pattern of muscle involvement localizes level.

Document myotomal and dermatomal findings.

Correlate with imaging.

SEP monitoring detects cortical ischemia during carotid or aneurysm surgery.

MEP monitoring assesses corticospinal integrity during spine surgery.

Warning criteria: SEP >50% amplitude drop or >10% latency increase; MEP >50–80% amplitude reduction.

Anesthetic effects: propofol and opioids preserve MEPs better than volatile agents.

Neuromuscular blockade abolishes MEPs; minimize.

Multimodality monitoring reduces false negatives.

Verify technical causes before attributing to neural injury.

Document baseline and changes.

Wallerian degeneration follows axonal injury; conduction fails distal to lesion within days.

Reinnervation proceeds at roughly 1 mm/day.

Tinel sign advances as regeneration proceeds.

Demyelination: slowed velocity, conduction block.

Axonal loss: reduced amplitude.

Compression: ischemia and focal demyelination.

Early decompression restores conduction.

Document injury pattern and prognosis.

Electrode impedance <5 kΩ; balanced impedances improve CMRR.

Clean skin and conductive paste reduce impedance.

Replace high-impedance electrodes.

Ground electrode between stimulator and recording electrodes.

Surface electrodes for NCS; needle electrodes for EMG.

Single-use needles reduce infection risk.

Document electrode type and placement.

Check reproducibility of responses.

Screen for epilepsy, prior head trauma, and medications lowering seizure threshold.

Contraindications: implanted electronic devices, intracranial metal, cochlear implants.

Repetitive TMS can induce seizures; have emergency protocol.

Provide ear protection during TMS.

Avoid TMS in pregnancy without risk-benefit discussion.

Document screening and consent.

Trained personnel must be present.

Stop stimulation if seizure occurs and manage airway.

Myopathy: normal NCS; EMG shows short-duration, low-amplitude motor units and early recruitment.

Myasthenia gravis: repetitive nerve stimulation shows >10% decrement; single-fiber EMG shows increased jitter.

Lambert-Eaton syndrome: incremental response with high-frequency repetitive stimulation.

Botulism: presynaptic defect; decrement with repetitive stimulation.

Normal SNAP and CMAP in myopathy.

EMG distinguishes myopathic from neurogenic patterns.

Document repetitive stimulation protocol.

Correlate with clinical and serologic findings.